Tin-coupled polymers are known to provide desirable properties, such as improved treadwear and reduced rolling resistance, when used in tire tread rubbers. Such tin-coupled rubbery polymers are typically made by coupling the rubbery polymer with a tin coupling agent at or near the end of the polymerization used in synthesizing the rubbery polymer. In the coupling process, live polymer chain ends react with the tin coupling agent thereby coupling the polymer. For instance, up to four live chain ends can react with tin tetrachloride thereby coupling the polymer chains together.
The coupling efficiency of the tin coupling agent is dependant on many factors, such as the quantity of live chain ends available for coupling and the quantity and type of polar modifier, if any, employed in the polymerization. For instance, tin coupling agents are generally not as effective in the presence of polar modifiers. In any case, the actual number of live chain ends in the rubbery polymer is difficult to quantify. As a result, there is normally unreacted tin coupling agent left in the polymer cement after the coupling process has been completed.
The free tin coupling agent is then available to react with any active protons present in the polymer cement to form hydrochloric acid. For example, excess tin coupling agent can react with most hydroxyl group containing polymerization shortstops or moisture from the air. The acid generated can then cleave the tin-carbon bonds in the tin-coupled polymer. Undesirable polymer degradation is, of course, the result of the tin-carbon bonds in the rubbery polymer being cleaved. This polymer degradation is normally evidenced by a drop in the Mooney viscosity and molecular weight of the polymer.